system timer: fix crash in <100Hz system timer

The kernel has a divide by zero crash when trying to run the system timer
less than 100Hz.  The problem is x/(HZ/USER_HZ) and related.  Now
x*(USER_HZ/HZ) will be used if HZ<USER_HZ.

I'm running the Linux kernel under qemu and went to run a slower system
timer to take less CPU (and battery) on the host.  I found that the kernel
paniced under emulation because of a divide by zero in three places.  Here
is the patch.  The base git was updated today 01-05-2008.  I went for a
20Hz system time by adding config HZ_20 etc to kernel/Kconfig.hz.  With
this patch I verified the system timer by looking at /proc/interrupts.

[akpm@linux-foundation.org: partially clean up the macro maze]
Signed-off-by: David Fries <david@fries.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/acct.h b/include/linux/acct.h
index 302eb727ecb8..e8cae54e8d88 100644
--- a/include/linux/acct.h
+++ b/include/linux/acct.h
@@ -173,7 +173,11 @@ typedef struct acct acct_t;
 static inline u32 jiffies_to_AHZ(unsigned long x)
 {
 #if (TICK_NSEC % (NSEC_PER_SEC / AHZ)) == 0
+# if HZ < AHZ
+        return x * (AHZ / HZ);
+# else
         return x / (HZ / AHZ);
+# endif
 #else
         u64 tmp = (u64)x * TICK_NSEC;
         do_div(tmp, (NSEC_PER_SEC / AHZ));